U‐Pb and Sm‐Nd ages from the Alpine Schist, New Zealand

The peak conditions and timing of metamorphism have been determined for the early Paleozoic Greenland Group in south Westland, New Zealand. The Greenland Group, which is the most extensive pre-Cenozoic unit west of the Alpine Fault in Westland, is dominated by greenschist facies greywacke and argillite. However, in the Jackson River valley this unit comprises amphibolite facies pelitic and psammitic gneisses. Psammitic horizons are dominated by biotite, plagioclase and quartz. Pelitic horizons have similar mineralogy but also contain sillimanite and K-feldspar, as well as boudinaged K-feldspar-bearing leucosomes. Conventional geothermometry and thermodynamic modelling indicate that peak metamorphism occurred between 660 and 700°C and 3.5 and 6.0 kbar. The timing of this moderately high-T/low-P event is constrained to be mid-Paleozoic by U-Th-Pb dating of overgrowths on detrital zircon (334 ± 18 Ma) and confirmed to be Carboniferous by a single population of monazite grains (343 ± 3 Ma). Greenland Group metamorphism in the Jackson River valley may represent Carboniferous high crustal heat flow in the Gondwana margin hinterland while crustal thickening was occurring closer to the subduction zone.

Section: Introductionmentioning

confidence: 99%

Carboniferous metamorphism and partial melting of the Greenland Group in the Jackson River valley, south Westland

Palmer

Scott

Muhling

et al. 2015

“…The nature and timing of these younger metamorphic events are not well understood. Garnet grade metamorphic mineral assemblages in various portions of the Alpine Schist have given U-Pb, Sm-Nd and Lu-Hf ages that range from c. 100 to c. 70 Ma (Mortimer & Cooper 2004;Vry et al 2004). Volumetrically minor anatectic pegmatite dikes in the Alpine Schist have also given U-Pb ages of c. 70 Ma (Chamberlain et al 1995;Batt et al 1999).…”

Section: Alpine Schistmentioning

confidence: 99%

Lead isotope constraints on the origin of Cenozoic orogenic gold systems in the Southern Alps and northwestern Otago, South Island, New Zealand

Mortensen

Craw

MacKenzie

et al. 2010

Lead isotopic compositions were determined for sulphides from Pliocene-Pleistocene gold-bearing veins in the Alpine Schist and from Miocene gold-bearing veins and vein breccias from the Shotover-Macetown area in the northwest Otago Schist belt. The lead isotopic signatures are consistent with derivation of Pb in the vein minerals predominantly from metasedimentary rocks that underlie the region. Differences in Pb isotopic signatures between deposits are interpreted to result from lateral and vertical lithological variability within the source rock mass. The host rocks also contain metabasic rocks with N-MORB, E-MORB or within-plate basalt chemistry. However, the observed Pb isotopic signatures in the gold-bearing veins preclude incorporation of significant amounts of Pb from the metabasites. The Pb isotopic signatures of lamprophyre dikes that were intruded into the Otago Schist coeval with Miocene gold mineralisation are distinctly more radiogenic than those of the hydrothermal veins. Thus, although the lamprophyre dikes were emplaced into similar extensional structural sites to the gold-bearing veins, there was no genetic relationship between lamprophyre dikes and gold mineralisation.

“…The beginning of regional metamorphism for the Otago Schist has been inferred at c. 199 Ma (Adams et al 1985). Mortimer & Cooper (2004) suggest that the timing of highest grade metamorphic mineral growth in the Otago Schist was during the Jurassic. Little et al (1999) inferred that peak metamorphism in the Otago Schist occurred in the middle Jurassic (170Á180 Ma) and that the Otago Schist was held at mid-to-lower crustal depths until 135 Ma.…”

Section: Geological Settingmentioning

confidence: 99%

Cretaceous age, composition, and microstructure of pseudotachylyte in the Otago Schist, New Zealand

Barker

Sibson

Palin

et al. 2010

At Tucker Hill, in Central Otago, New Zealand, a series of pseudotachylyte veins are hosted in quartzofeldspathic schist. Chilled margins, microlites, flow banding, and the crystallisation of mineral phases absent from the host rock provide unequivocal evidence for melting during pseudotachylyte formation. Whole rock analyses of pseudotachylyte reveal c. 3 ) enrichment of K 2 O, Ba, and Rb, and similar depletion of Na 2 O, CaO, Sr, and Eu, as compared to host schist. Formation age of pseudotachylyte is 95.991.8 Ma as measured by total fusion 40 Ar/ 39 Ar analyses. Stepwise heating of pseudotachylyte matrix yields an excellently defined 40 Ar/ 39 Ar plateau age of 96.090.3 Ma. These well-defined ages are attributed to the presence of potassium feldspar, low abundance of inherited lithic material from the host rock, and few fluid inclusions containing extraneous Ar. We propose that formation of these pseudotachylyte veins was related to Cretaceous extensional uplift and exhumation of the Otago Schist.Keywords: pseudotachylyte; schist; Otago; Ar-Ar; geochronology; friction melting IntroductionThe presence of pseudotachylyte (former friction melt) in a fault zone is commonly attributed to frictional melting of rock during seismic slip (Sibson 1975). As such, pseudotachylytes are valuable indicators that an exhumed fault zone was seismically active. Pseudotachylytes have been described from inactive, ancient fault zones (e.g., Outer Hebrides Fault Zone: Sibson 1975;Maddock 1983;Kelley et al. 1994) and present day seismically active fault zones (e.g., the Alpine Fault: Sibson et al. 1981;Bossiere 1991;Warr et al. 2003). Determining the formation age of pseudotachylyte can reveal when a fault was seismically active, and provide information on the significance of a pseudotachylytebearing fault zone relative to the timing of other regional deformation events (Kelley et al. 1994;Magloughlin et al. 2001;Sherlock & Hetzel 2001;Mueller et al. 2002;Warr et al. 2003).In Central Otago, New Zealand, a series of pseudotachylyte veins are found in schist outcropping on Tucker Hill (1698 24?23??E; 458 15?16?S), near the township of Alexandra (Fig. 1). The pseudotachylyte veins are hosted in the garnet-biotite-albite zone of the greenschist facies of the Otago Schist, which forms the basement rocks of much of the Otago region (Mortimer 1993a, b). Tucker Hill is approximately 15 km to the east of the Cromwell Gorge Shear Zone and approximately 20 km to the south of the Rise and Shine Shear Zone, which are two Cretaceous age extensional shear zones (Deckert et al. 2002).Previous studies that have attempted to determine the age of pseudotachylyte utilising 40 Ar/ 39 Ar geochronology in various localities have met with varying degrees of success. In particular, accurate and geologically meaningful 40 Ar/ 39 Ar age determinations are negatively influenced by the presence of variable amounts of inherited crystals (i.e., incompletely melted) from host rocks (Magloughlin et al. 2001;Warr et al. 2007), argon loss by diffusion or alteration ...